Piezoelectric ceramics are commonly perovskite oxides represented by ABO3, such as lead zirconium titanate (PZT).
However, since PZT contains lead as the A-site element, its impact on the environment has been regarded as problem. Piezoelectric ceramics that use lead-free perovskite-type oxides are thus sought after.
Barium titanate is known as a piezoelectric ceramic material of a lead-free perovskite oxide type. PTL 1 discloses barium titanate prepared by a two-stage sintering method. PTL 1 describes that a dense ceramic having a maximum grain diameter of 5 μm or less and excellent piezoelectric properties can be obtained by sintering a nano-sized barium titanate powder by the two-stage sintering method.
However, according to the two-stage sintering method, the length of time the first sintering temperature is retained needs to be small. As a result, the temperature of the ceramic to be sintered becomes uneven and it has been difficult to reproduce high piezoelectric properties.
For example, if a barium titanate ceramic of a practical size is to be sintered, rapid heating and a retention time as short as about 1 minute do not render the temperature of the ceramic even. In other words, not all parts of the sintered ceramic take an ideal nanostructure and thus piezoelectric properties sufficient for replacing PZT have not been achieved.
Another approach is to improve the piezoelectric properties of barium titanate by increasing the size of crystal grains. PTL 2 discloses the relationship between the average grain diameter and piezoelectric constant of calcium-doped barium titanate ceramics. The relationship shows that the piezoelectric constant (d31) increases as the average grain diameter of the piezoelectric increases from 1.3 μm to 60.9 μm.
According to PTL 2, the average grain diameter of the ceramic is adjusted by adjusting the length of time of wet-mixing a calcined powder. In addition, the temperature of main sintering following preparation of the calcined powder is increased to also increase the average grain diameter of the ceramic.
However, increasing the average grain diameter of barium titanate ceramics by this method decreases the contact area between crystal grains. This has resulted in a decrease in mechanical strength of the ceramic and thus ceramic parts have been susceptible to cracking during forming processes and operation of piezoelectric elements.
In sum, barium titanate piezoelectric ceramics desirably achieve both excellent piezoelectric properties and high mechanical strength.
Resonator devices such as ultrasonic motors desirably exhibit a high mechanical quality factor Qm. For example, barium titanate can achieve high Qm when doped with a transition metal such as Cr, Mn, Fe, Co, Ni, or the like. However, elements such as Mn act as grain growth accelerators that induce abnormal grain growth as barium titanate is sintered. Accordingly, it is difficult to control the grain diameter by the techniques disclosed in PTL 1 and PTL 2.
According to NPL 1, the structural phase transition between the tetragonal structure and the orthorhombic structure of barium titanate occurs at about room temperature and thus different piezoelectric properties have been observed at the same temperature between when the temperature is increased and when the temperature is decreased in the temperature hysteresis of the piezoelectric properties. Due to this drawback, barium titanate has poor piezoelectric property controllability although it has high piezoelectric properties at room temperature, and it has been difficult to practically apply barium titanate to piezoelectric elements.